One typical field effect transistor inverter circuit uses the combination of a load device and a switching device with the drain of the load device connected to a first power supply VDD, the source of the load device and the drain of the switching device connected to an output terminal, the gate of the switching device connected to an input terminal, and the source of the switching device connected to a second power supply VSS. For n-channel devices, VDD is typically +5.0 volts, VSS is typically 0 volts (ground potential), the threshold voltage of the switching device is approximately +1.0 volt, and the impedance of the load device is substantially greater than the impedance of the switching device. An applied "1" input logic state having a potential level near +5.0 volts causes both devices to conduct and the output potential level to be close to 0 volts, a "0". An applied "0" input logic state having a potential close to 0 volts causes the switching device to be biased off, and the output potential level to be at or close to +5.0 volts, a "1". If the input "0" level is +1.4 volts, as might be the case of some worse case input "0" signal received from a TTL circuit, the impedance ratio of the two devices must be modified or the threshold voltage or the switching device must be modified to obtain the desired inversion function. Such an impedance ratio modification causes the "0" output potential level to increase and thus reduces operating noise margins. In addition, it degrades the output fall time. Modification of the threshold voltage can be acommplished by adding an ion implantation step to the fabrication process which increases the complexity thereof and thereby potentially decreases yield and increases cost.
It would be desirable to have a field effect transistor inverter-level shifter circuit which can accept input "0" levels of a magnitude above the threshold voltage of the switching device but does not suffer a significant increase in the output "0" potential level or require an extra processing step to modify the threshold voltage of the switching device.